1. Field of the Invention
The present invention relates to a plating apparatus and a plating method for forming a uniform plating layer on a semiconductor substrate.
2. Background of the Invention
FIG. 7A shows a face-up type plating apparatus of the prior art wherein surface of a wafer 101 is plated while being arranged to face upward, and FIG. 7B is an enlarged view of a sealing portion of the wafer 101. In the drawing, the numeral 1 denotes a wafer processing vessel, 2 denotes a plating solution nozzle, 2a denotes holes of a drain-board, 3 denotes a plating solution supply pipe, 4 denotes a plating solution discharge pipe, 5 denotes a drain pipe, 6 denotes a plating tank, 7 denotes a plating solution, 8 denotes an upper portion of the wafer processing vessel, 9 denotes a lower portion of the wafer processing vessel, 10 denotes a cathode contact, 11 denotes a sealing material, 12 denotes a nitrogen gas injection release, 14 denotes a mesh electrode, 16 denotes an auxiliary sealing material and 101 denotes a wafer.
In the plating apparatus described above, the plating solution 7 supplied through the plating solution supply pipe 3 is discharged through the plating solution discharge pipe 4, and is circulated throughout the period of plating process. A specified voltage is applied across the mesh anode electrode 14 and the wafer 101 via the cathode contact 10, thereby to form a plated coating on the wafer 101 surface. In such a face-up type plating apparatus, because the wafer surface is arranged to face upward, deposition of air bubbles onto the wafer surface can be prevented and plated coating of better quality can be formed in comparison to the face-down plating method where the wafer is arranged to face downward.
FIG. 8 shows a distribution of a plated coating thickness over the surface of a 4" wafer plated with Au in the above plating apparatus with a current density of 5 mA/cm.sup.2 for a plating time of 12 minutes, where a distance from the wafer edge is plotted along the axis of abscissa and plate coat thickness is plotted along the axis of ordinate. As is clear from FIG. 8, the plated coating thickness shows a W-shaped distribution which has a peak at the center of the wafer and increases toward the edge.
Through an investigation into the cause of such a distribution of the plated coating thickness, it was found that the distribution of the thickness is greatly affected by the distribution of the transported ions of the plating metal, which is determined by the flow velocity distribution of the plating solution, and the distribution of the electric field on the wafer surface. Specifically, in the plating apparatus described above, because a flow velocity of the plating solution is highest, and accordingly the transported quantity of ions of the plating metal is largest, at the center of the wafer, which is located just below the plating solution supply pipe 3, a plated coating is formed with the largest thickness at the center, while the electric field is concentrated at the edge which leads to the plated coating being formed with the second largest thickness along the edge of the wafer.
On the other hand, a method may be used in which as the flow velocity of the plating solution 7 supplied through the plating solution supply pipe 3 is slower, thereby reducing the distribution of the flow velocity of the plating solution on the wafer surface. However, when such a method is used, the plating solution 7 becomes stagnant locally on the wafer surface, resulting in lower quality of the plating.